Spin Echoes and Steady Bo Gradients

Since the fringe field gradient has a fixed strength at the position of the probe head, the only experimental parameters of the pulse schemes shown in Fig. 3 that can be varied are the time intervals between the RF pulses. That is, the echo attenuation by relaxation must be considered and compensated for. This can conveniently be done with the aid of self-compensating pulse schemes [14]. 

Some care must be taken with fringe field methods if motional averaging of dipolar interactions is incomplete. Echoes then tend to be modulated by the so-called dipolar correlation effect [15, 16]. One can account for this phenomenon by dividing the (normalized) echo amplitudes recorded with and without gradient at the same Larmor frequency and at the same pulse intervals [11,12]. Any influence by relaxation and the dipolar correlation effect can be eliminated in this way. 

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